I. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a perpendicular magnetic recording medium.
II. Description of the Prior Art
Magnetic recording has been based on magnetization along the longitudinal direction of the plane of the recording medium. However, when an attempt is made to perform high density recording in such a system, the demagnetizing field within the recording medium increases. Thus, it is difficult to achieve very high density recording with this recording system.
In order to eliminate such a difficulty, a perpendicular magnetic recording system has recently been proposed which is based on magnetization along the perpendicular direction to the plane of the recording medium. In such a recording system, the demagnetizing field within the recording medium decreases when the recording density is increased, so that this system is quite suitable for high density recording. With the perpendicular magnetic recording system, it is necessary that the axis of easy magnetization be normal to the surface of the recording medium. Among such recording media, there is known a recording medium which is obtained by mixing magnetic particles with a binder, coating the mixture on a nonmagnetic tape, and introducing the tape into a magnetic field such that the plane of the tape is normal to the direction of the magnetic field. As a result, the axis of easy magnetization of the magnetic particles aligns along the direction of the magnetic field. After drying, a recording medium suitable for perpendicular recording is obtained.
As the magnetic particles, hexagonal ferrites such as barium ferrite (BaFe.sub.12 O.sub.19) are usually used. These hexagonal ferrite particles are in plate form and the axis of easy magnetization is normal to the plane of the surface so they are advantageous in that perpendicular orientation may be easily accomplished by magnetic field orientation processing. However, in order to use the hexagonal ferrites as a perpendicular magnetic recording powder, certain conditions must be satisfied.
For example, the hexagonal ferrites are too high in coercive force iHc (generally over 5,000 oersteds) to be recorded by the usual magnetic head. Thus, it is necessary to reduce the coercive force to a value suitable for vertical magnetic recording.
Further, it is preferred that the crystal size of the hexagonal ferrites be controlled within a range of 0.01-0.3 .mu.m for perpendicular magnetic recording. When the crystal size is less than 0.01 .mu.m, the ferromagnetism necessary for magnetic recording is not obtained, and when it exceeds 0.3 .mu.m, high density magnetic recording cannot be achieved. When the particle size is 0.2 .mu.m or less, particularly good results can be obtained.
It is further required that the hexagonal ferrites be homogeneously dispersed in a medium such as a binder or paint. Therefore, it is necessary that individual ferrite particles not aggregate at least during preparation thereof.
A hexagonal ferrite having a relatively low coercive force, good dispersibility and the desired particle size can be obtained by a glass crystallization technique, using hexagonal ferrite-forming components including a coercive force-reducing element as a starting material together with a glass-forming component.
However, when the magnetic particles of this type are dispersed within the binder, and the resultant mixture is applied to a substrate so as to perpendicularly orient the magnetic particles by means of the magnetic field, the following phenomenon often occurs. As previously described, when the substrate having a coating layer thereon is introduced into a magnetic field in a direction perpendicular to a direction of the magnetic flux of the magnetic field, the magnetic particles in the coating layer are oriented such that their axis of easy magnetization coincides with the direction of the magnetic flux. However, when the magnetic field is removed or the coated substrate is removed from the magnetic field after orientation of the magnetic particles, each magnetic particle serves as a very small magnet generating a magnetic field (demagnetizing field) applied in a direction opposite to that of the magnetic field for orienting the magnetic particles. If some magnetic particles are present which generate a demagnetizing field applied in a direction inclined by a given angle with respect to the vertical direction, then the perpendicularly oriented magnetic particles are subjected to torque by the demagnetizing field, thereby greatly impairing the perpendicular orientation of the magnetic particles. The recording characteristics of the perpendicular magnetic medium are greatly influenced by a perpendicular orientation ratio (%) (the average of direction cosines of easy axes of particles relative to perpendicular direction, or the ratio of the number of vertically oriented magnetic particles to the total number of magnetic particles); the higher the orientation ratio, the higher the reproduced output power and the recording density.